1. Field of the Invention
The present invention relates to an optical recording medium on which information is recordable by a laser beam, and an optical recording method.
2. Discussion of the Background
In recent years, attention has been drawn to shortening of the wavelength of a laser beam to be used for recording/readout, in order to improve the recording density of an optical recording medium.
An optical recording medium has been desired whereby recording and readout can be carried out by a laser beam having a shorter wavelength than the laser beam with a wavelength of 780 nm which is commonly used for CD-R or CD-RW, and DVD-R or DVD-RW has recently been developed whereby recording and readout are carried out by means of a semiconductor laser beam having a wavelength of from 640 to 680 nm.
Further, a laser having a wavelength of at most 600 nm, is now being developed as a technology indispensable for higher densification of an optical recording medium.
There are various types of optical recording media. Among them, an organic colorant type optical recording medium has characteristics such that it is easy to produce and inexpensive. As the colorant to be used for a medium to be recorded and readout by a short wavelength laser as mentioned above among such organic colorant type optical recording media, cyanine or phthalocyanine is, for example, proposed and disclosed in e.g. JP-A-6-336086, JP-A-7-161068, JP-A-7-262604, JP-A-7-125441 and U.S. Pat. No. 5,536,548.
Further, as an optical recording medium whereby recording and readout can be carried out by laser beams having two wavelengths of 780 nm and 640 nm, CD-RII has been developed and is disclosed, for example, in JP-A-7-276804, JP-A-7-156550, U.S. Pat. No. 5,633,106, JP-A-8-111034, JP-A-8-108623, JP-A-8-108624 and JP-A-10-74339.
It is believed that at the recorded region of such an organic colorant type optical recording medium, the optical properties have been changed not only by a change in the optical characteristics and a decrease in the layer thickness resulting from the thermal decomposition of the colorant, but also by a deformation due to softening and stress relaxation of the substrate which takes place subsequently, and a modulation for a record signal is thereby obtained.
Such a recording principle is the same as for CD-R. However, in order to have information recorded in a higher density by means of a laser beam having a shorter wavelength, it is required to form a recorded region which is smaller than the diameter of a beam to be used for recording and readout. To satisfy this requirement, a colorant material is required whereby the edge of a short mark is sharp and an optically large contrast can be obtained.
As an optical recording medium employing such a colorant material, the present applicants have previously proposed an optical recording medium disclosed, for example, in JP-A-9-58123.
For further development of the optical recording media in future, performance for high speed recording will be required in addition to the above-mentioned properties, and a colorant material having a high speed response to a laser beam for recording will be required. However, there have been no substantial proposals for colorant materials excellent in high speed response characteristics.
SPIE vol. 2514, 249 (1995) reports on the results of a study relating to the high speed response characteristics of colorants for CD-R, and it is disclosed that a phthalocyanine type colorant is superior to a cyanine type dye in the high speed response characteristics. Specifically, for example, at pages 254 to 255, a case is disclosed wherein the response speed of a colorant to a laser beam for recording was examined from the change (the inclination of a graph) in the reflectivity when a mark having a usual mark length was recorded on CD-R rotated at a low linear velocity (linear velocity: 0.2 m/s).
Heretofore, it has been believed in general that the response speed to a laser beam for recording, of a recording layer containing an organic colorant, is constant. Namely, it has been believed that the time (hereinafter referred to as the decay time) for the reflectivity to change and reach a saturated level is constant irrespective of the pulse length of irradiated laser beam, so long as a laser beam is irradiated for a sufficient length of time.
However, according to the measurements conducted by the present inventors, it has been found that the decay time changes as the pulse length of irradiated laser beam is changed, and, usually, the decay time prolongs, as the pulse length of irradiated laser beam is prolonged. The mechanism for such a change is not clearly understood, but it is believed that stress relaxation of the resin substrate contributes to the change in reflectivity, and a cause may be such that the critical temperature (such as the glass transition temperature) at which the resin substrate undergoes a deformation, depends on the frequency of the irradiation pulse.
Further, it has been found that the ratio of the decay time to the pulse length of irradiated laser beam, is more important than the decay time itself for the evaluation of the high speed response characteristic. The present invention has been accomplished on the basis of these discoveries.